The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services from geosynchronous orbit. A substantial number of radio frequency (RF) filters are required in such spacecraft. For example, a satellite input multiplexer may utilize a number of such filters, each filter having the functionality of separating and isolating a specific respective signal or bandwidth frequency from a broadband uplink signal received by a spacecraft antenna.
In the absence of the present teachings, the “tuneability” of such filters is limited. More particularly, when the center frequency of a bandpass filter is shifted, the bandwidth also changes. For example, referring to FIG. 1, it may be observed that a filter tuned to a center frequency of approximately 875 MHz exhibits bandpass characteristics 101, and has a bandwidth BW1 approximately equal to 40 MHz. When the same filter is tuned to a center frequency of approximately 960 MHz it exhibits bandpass characteristics 102, and has a bandwidth BW2 approximately equal to 55 MHz.
Such a substantial shift in bandwidth is unacceptable for, at least, satellite communications payloads, where constant, highly controlled bandwidth and spectral passband characteristics are required. The present invention is particularly useful for applications which require very precise bandwidth control, as for example in the case of input multiplexer channel filters employed in satellite communications payloads. Such bandwidth control has been mainly accomplished in the past by the use of fixed (non-tunable) filters.
Alternatively, when tuning filter resonant circuits by conventional means, it is possible to control bandwidth by tuning filter coupling elements as well the filter resonant circuits. Such an approach suffers from the disadvantage of introducing additional microwave components with the attendant undesirable extra volume, extra weight, cost and complexity.
A tuning filter topology utilizing cascaded, fixed, low pass and high pass filters tunable by a synthesizer and an upconverter/downconverter system has been used, for example, on the HYLAS satellite to tightly control composite filter bandwidth during the frequency adjustments. The disadvantage of this approach is that fixed low pass and high pass filters, due to their inherent very wide bandwidth, require many resonator elements to achieve necessary rejection characteristics with an attendant undesirable extra volume, extra mass and extra cost.
Thus, there remains an unmet need for an improved tunable filter topology which provides frequency and bandwidth tuning of the filter.